CD33 and Alzheimer disease: The relevance of neuroinflammation is further sustained by recent large-scale GWAS showing that the risk of developing late-onset AD is substantially more elevated in individuals with rare variants of microglial immunoreceptors: TREM2, encoding the triggering receptor expressed on myeloid cells 2 protein (89); CD33 (transmembrane receptor CD33), expressed on cells of myeloid lineage (90, 91); and PILRA (paired immunoglobulin-like type 2 receptor alpha) (92).